The determination of the speed of a test object takes place in the simplest way from the magnitudes to be defined--the path and the time. Either the time the test object needs to move the distance of a certain test stretch or the actual distance that is travelled in a certain time period is measured; only rarely are both magnitudes determined. Because time measurements are easier to carry out than distance measurements, the first of these options is usually preferred, especially when the trajectory of the test object is known in advance. In that case, one succeeds in monitoring the starting and finish points of a test stretch with devices that can control the presence of the test object at these points and with a suitable time-interval measuring instrument.
Although when measuring the speed only, a time measurement is taken, the other magnitude to be defined--the distance travelled--will also contain an error. Time measurements can be carried out today with a precision that well surpasses the accuracy needed here. It is much more difficult to define the test distance precisely enough, i.e, accurately and quickly enough to determine on which side of the starting or finish point of the test stretch the test object can be found.
Superficially viewed, the error in speed can be made as small as desired with a given absolute error of distance and time only if the test distance can be made large enough. This is, however, usually not practical, if only for the reason that the test object normally cannot be observed as long as one wants, and one usually cannot wait any amount of time for the test results.
Furthermore, measuring speed in the way discussed here requires that the test object does not change its speed when travelling the indicated distance or that it even stops after crossing the starting point and waits for a certain time before it continues to the finish point. When the test distance is extended, the probability that such an error will occur increases, and in addition, its consequences for the test result become more significant.
A further difficulty associated with putting the theory into practice results from the fact that several test objects can exist, whereby it is possible that at the beginning of a measurement--i.e., when a test object crosses the starting point of the test stretch--other test objects can be found on the test stretch--in other words, they have not yet crossed the finish point. In that case the measuring procedures for individual test objects can overlap in terms of time, and it is necessary to assign the events of crossing the starting and finish points of the test distance to the individual test objects in a reliable way. Doing so is demanding and fraught with error, and one strives to keep the test distance as short as possible--a circumstance that, in turn, requires the greatest possible accuracy when defining the starting and finish points.
These are conditions that affect the measurement of the speed of vehicles in street traffic. If one does not carry out the measurement at junctions or intersections--and also at parking lots--then the vehicles almost always move in well defined lanes and in one or two well defined directions. Lane changes can occur, but usually only small angular deflections from the normal direction of motion arise so that the errors in measurement that occur as a result of this remain small.
On the other hand, the special need for the shortest possible test distance exists because situations in which vehicles drive at low speeds at small intervals and must stop, unfortunately, occur more and more often. The speed test values, which are delivered by a measuring arrangement with a test distance along the order of several meters, under no circumstances possess any significance in such a situation.
But really high speeds can also occur. In the passing lane of a highway one usually does not drive faster than 200 km/hour. On the other hand, one encounters just as often the so-called stop-and-go traffic that occurs in traffic jams on highways, one reason why an extension of the test distance prevents an exact measurement of high speeds.
There therefore arises a problem of finding detection devices with which the starting and finish points of the shortest possible test distance can be monitored as accurately as possible--which means, in this case, that the two shortest possible sections of the trajectory must be checked for the presence of a vehicle on them. Moreover, the time difference between the actuation of the detection device at the start of the test distance and again at the finish must be determined as accurately as possible. A sufficient condition for this is the possibility of determining the exact time points of the actuation; but this must not be required. Only the time difference is required. That is of decisive significance for the test procedure proposed here.
One possibility of detecting vehicles is represented by the known induction loops embedded into the roadway. Because they are several meters long in the driving direction, a test distance of the short length required here therefore cannot be realized.
A further possibility is offered by radar and microwave measuring systems that emit rays that are reflected by the vehicle. Radar and microwave rays can be much more finely concentrated than an induction-loop detector can be extended; nonetheless, they still do not obtain a detection accuracy sufficient for the size of the test fields that are being aimed at here. The same goes for an ultrasound procedure.
In the case of detection devices on an optical basis, the desired accuracy can be obtained. Known procedures have, nevertheless, specific weaknesses in street traffic. Simple light barriers cannot be used. If one installs them horizontal to the sender on the one side of the traffic lane and the receiver on the other side, several traffic lanes are almost always monitored; in other words, several trajectories are monitored at the same time without there being a possibility of distinguishing between them. In addition, a light barrier used in such a way would only operate accurately if the vehicles possessed a well defined vertical front edge, similar to a ship's bow. In the case of modern passenger vehicles, a wedge shape is, however, preferred. In addition, every vehicle produces certain pitching motions. All of this hinders the accuracy of the detection considerably.
A measuring procedure, in particular for sensors of this type, is described in DE 30 22 345 A1. The document also shows some measures for overcoming the difficulties mentioned: measuring errors can occur when several vehicles in various traffic lanes pass the measuring device at the same time. For this reason, two measurements of the speed are carried out one after the other, and if conflicting values result, the measurement is ignored.
More advantageous would be light barriers that are installed vertically; in that case, individual traffic lanes could be monitored selectively, and well defined horizontal front edges exist in modern motor vehicles. In this case the sender must be attached above the street to the frame of a traffic sign or something similar, and the receiver must be embedded in the road surface, or vice versa. Such demanding installation impairs the use of a detection device, but in spite of this, the really decisive thing is the fact that the light path can be blocked by dirt.
As a remedy, reflection light barriers were suggested in which the sender and receiver are both installed over the roadway. In this case, the sender emits a narrow light beam; and the receiver is lined up with the light spot on the roadway produced by it and reacts to changes in its brightness in connection with the vehicle's lights instead of street lights. This design, however, strongly suffers from the fact that the brightness of the light spot as the vehicle drives in may not necessarily change (enough). Because the diffusely reflected light is very weak, a large amplification with a correspondingly high noise level is essential. Signal changes that do not stand out from this noise sufficiently occur too frequently to be able to designate such a reflection light barrier as reliable.
This problem can be met by installing an optical sensing device instead of a reflection light barrier, which can function both on a trigonometric basis or by measuring the light time delay. A vehicle that has just driven in covers the street to a certain height so that the range test value must change only as long as the light that comes on in connection with the receiver is sufficient for a range measurement. Unfortunately, this, too, cannot always be guaranteed.
Both of the above-mentioned approaches to the solution have, in addition, the disadvantage that only one point of the traffic lane is monitored--traffic lanes on which vehicles (especially narrow ones like, e.g., motorcycles) can simply pass by [unnoticed]. While the transmitted light beam can easily be widened transverse to the driving direction, the representation of the light strip produced by it on the receiver element is difficult and reduces the sensitivity of the detection device considerably.
Linear overrun sensors that are embedded in the road surface transverse to the driving direction and react to the pressure caused by the tires of a vehicle offer an alternative in which many of the above-mentioned problems are avoided. As a consequence, a monitoring area can be created that covers only about 1 cm in the driving direction, but, however, includes the entire width of the traffic lane in the transverse direction, whereby in the case of several traffic lanes, each individual one can be monitored selectively.
The consequence for the measurement of the speed is that small test stretches can be created, whereby the trajectory of the test object may shift considerably--above all longer than it is wide. Problems with it getting dirty hardly ever arise: only heavy snow or layers of ice hinder its sensitivity. Although one must mill a slit in the road surface to install it, the installation is made much more simply than that of an induction loop, particularly for measurements of speed that require several detection devices.
Such linear overrun sensors are, for instance, the known piezo-pressure tubings, which have already been used to measure speed. Recently, however, sensors on the basis of a fiber-optic light guide have become generally known and are also excellently suited to this purpose.
The document WO 87/063 49 A1 describes a measuring arrangement with such linear overrun sensors that is also on a basis of a fiber-optic light guide. It is proposed that two sensors be connected with each other so that a solid interval can be maintained between them, and then that this arrangement be installed transverse to the traffic lane. As a result, the measurement arrangement becomes portable.
One particular difficulty with the use of linear overrun sensors consists in the fact that the signals, which are produced by running over several sensors, are similar qualitatively, but nonetheless usually have different amplitudes, if only because one can only rarely guarantee that the sensors are always carrying equally heavy traffic. So that this idiosyncrasy does not cause errors in measurement, special measures must be taken.